Sundry gas mixtures are available from natural sources as well as in products and by-products of known chemical processes. Various techniques have been employed or proposed for recovery of one or more individual components of such gas mixtures, including among these: adsorption in selective solvents, cryogenic fractionation, selective adsorption by solid adsorbents, and certain combinations of such unit operations.
Natural gas, for example, typically comprises in the order of from about 50 to about 95 volume percent methane, with about 4 or 5% and up to 40-50% C.sub.2 to C.sub.6 hydrocarbons, up to about 5 to 6% nitrogen and the remainder made up chiefly of carbon dioxide (less than about 1%) and other impurities (less than about half percent). Among known industrial uses for natural gas, other than its use as fuel, is in the production of hydrogen and other valuable products therefrom.
Among conventional processing of natural gas or other methane-rich gas mixtures is that of reforming, particularly that carried out by reaction with steam, according to the equations: EQU CH.sub.4 +H.sub.2 O.fwdarw.CO+3H.sub.2 EQU CH.sub.4 +2H.sub.2 O.fwdarw.CO.sub.2 +4H.sub.2
The separation of hydrogen from oxides of carbon in the reformate by present technology is costly from the standpoint of initial capital investment as well as operating costs incurred in the energy intensive methods employed. Conventional processes generally combine treatment of the starting mixed gas charge with a selective solvent absorbent, such as monoethanolamine (MEA), to remove the bulk of the CO.sub.2 followed by cooling to cryogenic temperatures for separation of the carbon monoxide from hydrogen. Regeneration of the absorbent requires considerable energy input, which is largely obtained from process heat that would otherwise be available for generation of steam. Since small amounts of both water and CO.sub.2 still remain in the process gas effluent from the absorber, it is necessary to incorporate driers in the system to prevent these species from freezing out in the gas chilling equipment (coldbox). These driers further intensify the already significant capital costs.
As illustrated in FIG. 1, the conventional "state of the art" system for separation of H.sub.2 /CO/CO.sub.2 derived from steam reforming of a methane-rich gas mixture entails the use of a solvent absorbent (generally monoethanolamine) for removal of CO.sub.2, a so-called "coldbox" for bulk separation of the carbon monoxide from its admixture with hydrogen, and a solid adsorbent unit (PSA) for further purification of the hydrogen. The crude hydrogen product obtained form the coldbox is about 98% pure and may be brought to about 99.9% purity in the PSA unit. The described operation typically provides about 98% hydrogen recovery, the reject gas being utilized as fuel in the reformer furnace.